Generally, a light sensitive component in an illuminance meter uses a silicon diode with filters for light-measuring levels ranging from 0.1 Lux to 200,000 Lux. The illuminance meter was used very well in diffuse light conditions. However, for measurements in direct sunlight, there are very large differences depending on the angle of a photo detector. If the angle of the photo detector is changed by 1 degree at clear sky conditions towards the direction of the sun, measured values may differ from each other by more than one order of a magnitude. Therefore, reproducible measurements of the direct radiation are impossible if the angle, at which the illumination is measured, cannot be complied precisely.
Usual illuminance meters are mostly used for indoor purposes at typical lighting conditions between 100 and 2000 Lux. The maximum factor for such light conditions indoors is 20. For such lighting conditions, constructions of commercial photo detectors are designed. The housing of such photo detectors are made of plastic and are very light. If measurements are carried out directly behind a glass of a window, a factor is taken into account, which is at least ten times higher than normal indoor measurements. If there is a slight deviation at a measuring point, in particular with regard to a setting angle, then the measurement results are affected dramatically. With the commercial design of photo detectors, the measurements may not be carried out satisfactorily. The low weight of the photo detectors in addition to an only millimeter-wide supporting surface of the photo detector may slip very slightly. Due to slipping, there could be a change in the setting angle with respect to the direct rays of the sun. Accordingly, the measurements may have a very high margin of error and may become useless for valid evaluations.
Thus, there is a need for an improved illumination measuring device to enable precise measurements of the incident light even under direct sunlight.